As global temperatures rise and our oceans heat up, we can expect more and more bleaching to occur at coral reefs around the world. But what are coral reefs and why are they important? And what does it mean when scientists say they are bleaching?

Coral reefs are a hard, rocky material that forms as a byproduct of the life processes of corals, organisms that live together in enormous marine communities. These reefs grow just a few centimeters each year, the U.S. National Ocean Service says, so it has taken a long time for the biggest ones to reach their massive size.

“It can take up to 10,000 years for a coral reef to form from a group of larvae,” the organization says. “Depending on their size, barrier reefs and atolls can take from 100,000 to 30 million years to fully form.”

Reefs are classified as a barrier or an atoll depending on their formation. Barrier reefs circle the shoreline of a landmass but are a certain distance from the land, while atolls have no visible land enclosed within them because it has become submerged.

All sorts of creatures work together to make a coral reef the special place it is. Scientists and divers can spot fish, sponges, crustaceans, turtles, algae and tons of other species, large and small, that find shelter and food at coral reefs.

“In addition to being some of the most beautiful and biologically diverse habitats in the ocean, barrier reefs and atolls also are some of the oldest,” the National Ocean Service says.

But these reefs are in danger. Coral reefs bleach as oceans get warmer — expelling the algae living inside them and turning white. In this state, they are more vulnerable to disease.

“When a coral bleaches, it is not dead,” the ocean service explains. “Corals can survive a bleaching event, but they are under more stress and are subject to mortality.”